1. Field of the Invention
The present invention relates to an auto-focusing optical system, and more particularly, to an auto-focusing optical system for a camera module, capable of obtaining a high resolution in a small size using a liquid lens whose curvature is varied depending on an applied voltage.
2. Description of the Related Art
Generally, a camera has a plurality of lenses and is configured to adjust an optical focal length by moving each lens to change a relative distance between lenses. Recently, a cellular phone with a built-in camera has emerged and accordingly it has become possible to photograph a still image or a moving picture using the cellular phone. Further, a recent trend is that performance of a camera is getting gradually improved in photography of high resolution and quality.
FIG. 1 is a perspective view of a conventional camera module which does not have a focusing function.
Referring to FIG. 1, the conventional camera module is configured to include an image sensor 54, a filter assembled to a lower side of a housing 51, and a plurality of lenses mounted inside a lens barrel 52.
The lens barrel 52 is fixed in the housing 51 using epoxy after a focal length of a lens array 53 is adjusted to the image sensor 54 using screw threads formed on an inner periphery of the housing 51 and an outer periphery of the lens barrel 52.
However, since it is impossible to focus at a specific distance in such a fixed focusing type, there is a limitation in sharpness of image quality.
Therefore, a focusing function is indispensably required for a camera module of more than mega pixels.
For that purpose, a necessity for application of a camera module having an auto-focusing unit, a close-shot unit, and an optical zoom unit on a cellular phone has emerged, but there has been difficulty in mounting a camera module manufactured according to the related art on a small-sized cellular phone.
That is, the camera module of the related art has used a DC (direct current) motor as an operating source, for performing a focusing and/or a zooming by changing a relative distance between an image sensor and a lens. However, according to the camera module of the related art, since a plurality of reduction gears are connected together so that a relative distance between lenses may be changed, it is difficult to accurately control a position of a lens so as to precisely perform a focusing due to reduction in response time and deviation in rotational speed. Further, since the camera module is big in its size and complicated in its structure, it is difficult to realize an auto-focusing function at an extremely limited space in an inside of a small-sized optical apparatus such as a cellular phone.
Further, since a plurality of lenses is used to obtain a high resolution, manufacturing cost is increased and mechanical operation is required, which increases power consumption.
In the meantime, PCT international publication No. WO 03/069380 discloses a variable focus lens.
FIG. 2 is a schematic, cross-sectional view of a variable focus lens suggested as an embodiment by WO 03/069380.
Referring to FIG. 2, the variable focus lens includes: a fluid chamber 65 of a cylinder shape having a cylinder wall, and containing a first and a second fluids A and B having different indexes of refraction, respectively, in which the first and second fluids A and B are and not mixed with each other, and contact each other through a meniscus 74; a fluid contact layer 70 arranged in an inside of the cylinder wall; a first electrode 62 separated from the first and the second fluids A and B by the fluid contact layer 70; and a second electrode 72 for activating the second fluid B.
Here, the first electrode 62 is of a cylindrical shape and coated by an insulating layer 68, and made of metal material, and the second electrode 72 is arranged on one side of the fluid chamber 65.
Further, a transparent front element 64 and a transparent rear element 66 constitute a cover part of the fluid chamber 65 for accommodating the two fluids.
Operation of the variable focus lens having the above-described construction will now be described in the following.
If a voltage is not applied between the first and the second electrodes 62 and 72, the fluid contact layer 70 has a high wettability for the first fluid A rather than the second fluid B.
If a voltage is applied between the first and the second electrodes 62 and 72, wettability for the second fluid B is changed due to electrowetting, and contact angles Q1, Q2, Q3 of the meniscus 74 are changed as shown in FIG. 2.
Therefore, a shape of the meniscus is changed depending on an applied voltage and a focusing is performed using the change generated in the shape of the meniscus.
That is, as shown in FIGS. 2A, 2B, and 2C, an angle between the meniscus 74 and the fluid contact layer 70, measured by the first fluid A is changed into about 140°, 100°, 60° depending on intensities of the applied voltage.
Here, FIGS. 2A, 2B, 2C represent arrangements in cases of a high negative power, a low negative power, and a positive power, respectively.
As described above, the variable focus lens using the fluid (referred to as ‘liquid lens’) has an advantage in manufacturing a small-sized product compared to the related art in which a focusing is performed using mechanical operation of a lens.
However, since a resolution is limited to about three hundred thousand pixels and a high resolution cannot be obtained in case of using an only liquid lens, there is limitation in applying the liquid lens to a current camera module of mega-pixel class.
To solve such problems, an auto-focusing optical system that can be manufactured in a small size, is highly required.